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The activities of a living cell depend on the ability of its membrane to
1. transport raw materials into the cell (and out of)
2. transport manufactured products and wastes out of the cell
3. prevent unwanted matter from entering the cell
4. prevent the escape of matter needed to perform cellular functions
each phospholipid molecule has a head that is hydrophilic (water-loving) and two tails that are hydrophobic (water-fearing)
Proteins in membranes serving the following functions
a) moving substances across the membrane
b) carrying out chemical reactions (they act as enzymes)
c) some have "marker" molecules (carbohydrate chains) on their surface allowing cells to recognize each other
d) allow messenger molecules (such as hormones) to attach
e) assist in cell-to-cell communication and control of cell functions
Protein position within a membrane:
Peripheral proteins are partially embedded in the inside or outside surface of the membrane
Protein position within a membrane:
Integral proteins extend through the entire bilayer and project from both surfaces
The Fluid-Mosaic Model
Cell membrane molecules are in constant motion (drifting past each other) resulting in:
a) membrane flexibility
b) cell's ability to change shape
Cell Membrane Function: A Biological Barrier
a cell membrane prevents many materials from entering the cell.
Cell Membrane Function: 6 materials a membrane prevents from entering the cell
- salts - atoms - viruses
- sugar - ions - bacteria
- proteins (7? this is what was in the notes)
most organelles are surrounded by membranes with the same structure as a cell membrane
Cell Membrane Function: Apoptosis
when the lysosome bursts and releases it's digestive enzymes into the cell resulting in cell destruction
Cell Membrane Function: A Selective Filter
Cell membranes are semi-permeable allowing some materials to cross, while excluding others. They can select a) by particle size
- small enough to enter membrane - O2 H2O
- too large to cross - sugar
b) particular materials to transport across (they bind to chemicals based on their size shape or charge)
Transport Across Cell Membranes: Selective Transport
the movement of only certain substances across the cell membrane
Transport Across Cell Membranes: Concentration Gradient
the difference in concentration between two areas for any given molecule produces a gradient or path of movement in which molecules move toward areas where the concentration of particles is lower
- molecules move down a concentration gradient
Transport Across Cell Membranes: Equilibrium
a state at which molecules are evenly distributed (the concentration is equal throughout the medium)
- molecules continue moving but equilibrium is maintained
Types of Transport Across Membranes: Passive Transport
movement across cell membranes without an input of energy
2 reasons molecules move.
1. Brownian Motion
2. Concentration gradients
Three Types of Passive Transport: 1. Diffusion
the net movement of particles from an area of high concentration to an area of low concentration
- no energy is expended
- in a cell very small particles can cross the cell membrane by moving between the phospholipid molecules
Three Types of Passive Transport 2.Osmosis
the diffusion of water molecules across a membrane (water molecules move from where they are more highly concentrated to where they are less concentrated)
has the same solute concentration on both sides of the cell membrane. Equilibrium has been reached. EQUAL FLOW of water into and out of the cell
The cell wall of a plant resists the pressure of a water-filled vacuole keeping the plant firm
Three Types of Passive Transport 3.Facilitated Diffusion
diffusion of molecules across the cell membrane by way of transport proteins.
have 3-D shapes that make them highly selective, recognizing atoms or molecules by shape, size or charge.
Two types of transport proteins
a) carrier proteins - facilitate the diffusion of glucose across the cell membrane
b) channel proteins - have tunnel-like pores filled with water that allow charged ions in and out of the cell
the movement of molecules and ions against the concentration gradient which requires ATP energy and carrier proteins to pump these molecules from an area of low solute concentration to an area of high solute concentration.
the use of vesicles to facilitate movement of substances that are too large to enter or exit the cell via transport proteins
the cell membrane forms a pocket around the material to be transported, then either pinches off as a vesicle or a vacuole.
Differentiate between a Vacuole & Vesicle.
Vesicle transports contents
Vacuole stores the ingested material
Two types of Endocytosis:
a) phagocytosis - when cells "eat" by taking in large particles or other cells
b) pinocytosis - when cells "drink" by taking in droplets of fluid
Receptor - mediated endocytosi
receptors, like antennae, detect specific compounds or cells and bind with them, triggering endocytosis.
the reverse of endocytosis, whereby the membrane of vesicles or vacuoles fuses with the cell membrane and the stored contents are expelled from the cell.
Membranes at Work 1.Water Purification: Reverse osmosis
uses pressure to force contaminated water through a membrane with fine pores that will not allow bacteria, salts, and other dissolved molecules through, resulting in water with fewer impurities
Membranes at Work 2. Kidney Dialysis
filters toxic wastes that accumulate in the blood while retaining necessary proteins, glucose, amino acids & ions
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